1. Field of the Invention
The present invention is related to a photo relay, particularly to a photo relay with two metal-oxide-semiconductor (MOS) transistors to control the output circuit thereof.
2. Description of the Prior Art
A relay is an electrically operated device with a control system (also known as an input circuit) and a system to be controlled (also known as an output circuit), and is usually used in the automatic control system. The relay is operated as an “automatic switch” to control a large current by using a small current and has the abilities including automatic control, overload or fault protection, signal conversion, etc.
FIG. 1 is a circuit diagram of a conventional photo relay. The photo relay 10 includes an illuminating unit 12, a photoelectric conversion unit 14, and two metal-oxide-semiconductor (MOS) transistors 16, 18. The illuminating unit 12, such as a light emitting diode (LED), is connected to the input ends T1, T2 of the photo relay 10 for receiving a current signal (i.e. the small current) and generating an optical signal (usually an infrared light) based on the current signal. The photoelectric conversion unit 14 includes a photodiode array (not shown). The source electrodes S1, S2 of the two MOS transistors 16, 18 are connected to each other, and the drain electrodes D1, D2 of the two MOS transistors 16, 18 are connected to the output ends T3, T4 of the photo relay 10. After receiving the optical signal from the illuminating unit 12, a voltage change (i.e. a voltage drop) would be induced to change the conduction state of the two MOS transistors 16, 18 and control the current (i.e. the large current) flowing through the two MOS transistors 16, 18.
FIG. 2 is a cross-sectional schematic view of a conventional packaging structure for the photo relay 10 in FIG. 1, and FIG. 2A is a top schematic view showing the photoelectric conversion unit 14 and the two MOS transistors 16, 18 in the lower portion of the packaging structure. Please refer to FIGS. 2 and 2A for the arrangement of the photo relay 10 in the packaging structure The illuminating unit 12 is usually located in the upper portion of the photo relay 10, and the photoelectric conversion unit 14 as well as the two MOS transistors 16, 18 are located in the lower portion of the photo relay 10. In general, the photoelectric conversion unit 14 and the two MOS transistor 16, 18 are separated chips. The illuminating unit 12 is aligned with the photoelectric conversion unit 14, especially the photodiode array (not shown) thereon. The photoelectric conversion unit 14 has at least two output ends electrically connected to the gate electrodes G1, G2 of the two MOS transistors 16, 18 by using bonding wires 19 for controlling the conduction state of the MOS transistors 16, 18. In addition, please referring to FIGS. 1 and 2A, the drain electrodes D1, D2 of the two MOS transistors 16, 18 are located at the bottom of the chips and act as the output ends of the photo relay 10 by connecting to the two pins T1, T2 of the lead frame directly, and the source electrodes S1, S2 of the two MOS transistors 16, 18 are located at the upper surface of the chips and connected with each other by the conductive wire 20.
However, as shown in FIG. 2A, it is needed to preserve a certain area on the lead frame in the lower portion of the packaging structure of the photo relay 10 to locate the three individual electric components or chips (i.e. the photoelectric conversion unit 14 and the two MOS transistors 16, 18). The packaging size of the photo relay would be restricted by the limitation.